Existing hydraulic steering systems with a supply connection arrangement, have a high-pressure connection and a low-pressure connection, with a working connection arrangement, a directional valve and a metering pump unit being arranged between the supply connection arrangement and the working connection arrangement; a metering pump arrangement having at least two hydraulically parallel-connected and mechanically parallel-operated metering pumps with a shut-off valve in a hydraulic connection between the two metering pumps; with the shut-off valve having a return spring and a control inlet that is connected with the high-pressure connection.
A hydraulic steering system of this kind is disclosed in DE 195 11 501 A1.
A steering system of this kind is preferably used for the steering of vehicles. Both the metering pumps and the directional valve are connected with a steering handwheel or a similar device. When the steering handwheel is turned, the directional valve is displaced in the desired direction, and the metering pump arrangement supplies hydraulic fluid until a steering motor connected to the working connections of the working connection arrangement has reached the desired position. During normal, undisturbed operation, as long as hydraulic fluid with a sufficient pressure is available at the high-pressure connection, both metering pumps are active. They can supply a correspondingly large volume of hydraulic fluid, which enables a fast reaction of the steering motor to movements of the steering handwheel.
When the pressure at the high-pressure connection drops, for example because of a defect in the hydraulic supply, the hydraulic steering can no longer be operated through support from this hydraulic pressure. In this so-called xe2x80x9cemergency operationxe2x80x9d the metering pumps are also used as auxiliary pumps, that is, the metering pumps are used for producing the pressure of the hydraulic fluid. However, for this purpose, the required energy must be provided via the steering handwheel, that is, by human muscular strength. Therefore, by means of the shut-off valve, the connection between the two metering pumps is interrupted. One of the metering pumps will be short-circuited, and the operator now only has to activate the other metering pump. To achieve the same deflection of the steering motor, the operator will have to turn the steering handwheel.
In the known hydraulic steering system, the shut-off valve between two metering pumps is acted upon by pressure from the high-pressure line and in the other direction by the force of the return spring and the pressure in the low-pressure line. However, to an increasing extent, additional consumers (or xe2x80x9cloadsxe2x80x9d) are connected to the low-pressure line, which can work with a lower pressure. This measure serves the purpose of saving energy. When the steering is not activated, that is, the hydraulic steering system is not active; the hydraulic fluid simply flows through the steering system. Via a usually available load-sensing system (LS-system) the supply pressure is correspondingly reduced, so that in the low-pressure line, also called tank line, merely a lower pressure of, for example, 15 to 25 bars is effective, with which a coupling or a brake can be activated. When the steering system is activated, however, a higher supply pressure is available. A large share of this pressure is, however, xe2x80x9cconsumedxe2x80x9d in the steering system, so that a correspondingly reduced pressure is again ruling in the tank line.
It is, however, a condition for using the low-pressure or tank line for supplying a low-pressure hydraulic that the first steering inlet of the shut-off valve is acted upon by a correspondingly higher pressure. This pressure is produced in that a prestressed non-return valve is arranged after the branch of a steering pressure line to the first steering inlet of the shut-off valve. This non-return valve ensures that a differential pressure of 8 to 10 bars exists over the shut-off valve, that is, the pressure on the side of the first steering inlet is by 8 to 10 bars higher than the pressure in the low-pressure line. This partially causes substantial energy consumption, as the hydraulic fluid having a correspondingly high pressure must pass the highly prestressed non-return valve.
It is therefore a principal object of this invention to reduce the energy consumption when using the steering system in connection with a low-pressure connection, which is loaded by consumers.
The problems of existing hydraulic systems as mentioned above are solved in that the shut-off valve has a second steering inlet, which can be acted upon by a pressure from a lowest-pressure connection.
Thus, next to the low-pressure connection, which is also loaded by consumers, a lowest-pressure connection is used, with which it is ensured that it always carries a relatively low pressure. Usually, this is the tank pressure, the atmospheric pressure or another pressure, also called T0. Still, also even lower pressures can rule in many hydraulic systems, as long as it is ensured that no significant forces are acting upon the shut-off valve through the pressure at the lowest-pressure connection. The pressure at the first steering inlet merely has to overcome the force of the return spring. Accordingly, the pressure difference over the shut-off valve can be kept small. Energy losses are substantially reduced.
The pressure at the second steering inlet acts in the same direction as the return spring. Then, when desired, the effect of the return spring can be supported.
Also, a non-return valve prestressed with low opening pressure is arranged in high-pressure line connected with the high-pressure connection, a steering line branching off to the first steering connection in front of said valve. This non-return valve is not required to build up a pressure over the shut-off valve, with which pressure the shut-off valve is switched to one of its two working positions, but serves as a block to prevent retroaction from the steered wheels on the hydraulic system, the so-called xe2x80x9ckick-backxe2x80x9d. For this reason, a high prestressing is not required either. On the contrary, this non-return valve can be prestressed with such a small force that merely a closed state is ensured, and no larger forces are required to open it. Accordingly, also the pressure drop at such a non-return valve is small. The energy consumption is kept low.
It is particularly preferred that the opening pressure is maximum 1 bar. Compared with the known case, this is relatively little. Usually, an even smaller pressure will be sufficient, for example 0.5 bar. In the known case, this pressure would be in the range around 10 bars.
Preferably, the second steering inlet is connected with a changeover valve, which connects the second steering inlet with either the lowest-pressure connection or the high-pressure connection. Thus, the second steering inlet can additionally be used for a deliberate switching from the use of both metering pumps to the use of one metering pump. Thus, the transmission ratio between the operation of the steering handwheel or a similar manipulator and the steered wheels can be changed. For example, when driving on a public road, it is possible to use only one metering pump, that is, a smaller displacement, so that the deflection of the steered wheels is made safer and less sensitive. When, however, the vehicle is driven offroad, for example on a building site, it may be expedient to use both metering pumps. In this case the same deflection of the steered wheels is achieved already with small deflections of the steering handwheel. Thus, the shut-off valve gets an additional purpose, namely selecting the xe2x80x9ctransmission ratioxe2x80x9d between the steering handwheel and the steered wheels.
It is also preferred that the changeover valve can be remote-controlled. Thus, the changeover valve can be arranged near or even in the steering system, without negative influence on its operational qualities. The changeover valve can, for example, be activated from the area of the driver seat.
The changeover valve is a solenoid valve. A solenoid valve is a particularly simple embodiment of a remote-controllable valve.
The disconnectable metering pump has at least threefold the displacement of the other metering pump. Accordingly, a transmission ratio of at least 1:4 occurs, when the operation with both metering pumps is compared with the operation with only one metering pump.
Also, the shut-off valve has a slide, which, together with a distributor plate closed in the area of the slide and a housing surrounding the slide, borders a low-pressure chamber. The relation of the pressure in the low-pressure chamber to the pressure at the first steering connection thus determines the position of the slide. The distributor plate is closed at least in the area of the low-pressure chamber, that is, it creates a genuine border surface for the low-pressure chamber. Accordingly, the pressure in the low-pressure chamber is not influenced by any pressures in the metering unit, which is separated from the low-pressure chamber by the distributor plate.
Preferably, the low-pressure chamber is sealed in relation to all hydraulic lines. In this case, the second steering inlet can even be opened towards the surroundings, that is, under these circumstances atmospheric pressure rules in the low-pressure chamber. Accordingly, only a relatively low pressure is required to move the slide.
It is preferred that the slide has a seal, which is displaceable together with the slide in relation to the housing. This seal prevents the hydraulic fluid from penetrating past the slide.
A lowest-pressure channel is provided in the housing, which ends in the low-pressure chamber in the area of the distributor plate. Via the lowest-pressure channel the pressure from the lowest-pressure connection reaches the low-pressure chamber. As the lowest-pressure channel ends in the area of the distributor plate, the low pressure can practically act upon the slide over the complete movement stroke of the slide.
In addition, a recess surrounding the low-pressure chamber is arranged between the distributor plate and the housing, the lowest-pressure channel ending in said recess. In this case, the lowest-pressure channel can no longer be closed by the slide. In all positions of the slide it is ensured that the slide is acted upon by the pressure in the lowest-pressure channel.
A first section of the lowest-pressure channel extends perpendicularly to the distributor plate. This facilitates the manufacturing.
The lowest-pressure channel has a second section, which extends radially to the movement direction of the slide. This also facilitates the manufacturing. The lowest-pressure channel then extends at a right angle.